1. Field of the Invention
The present invention relates to an optical scanning device used as an optical system for writing in an image forming apparatus such as a printer, a facsimile, a copier, and the like and relates to an image forming apparatus having the same.
2. Description of Related Art
In recent years, there have been developed optical scanning devices using a laser beam as a light source, such as multi-beam optical scanning devices that write a plurality of lines simultaneously by using a plurality of laser beams. In particular, since a semiconductor laser (a laser diode) is easily controlled to turn ON/OFF and is compact in size, optical scanning devices using such a semiconductor laser as a light source have come to be developed increasingly eagerly.
In such an optical scanning device, a light beam emitted from a light source such as a semiconductor laser is deflected by deflecting means such as a polygon mirror, and is then made to scan across a photoconductor. In this way, an electrostatic latent image is formed. Additionally, in such an optical scanning device, in general, diffused light emitted from the light source is converted into parallel ray by means of a collimator lens, and the parallel ray thus obtained is faired by an aperture stop (hereinafter referred to as an “aperture”) to obtain a light beam having a desired beam spot diameter.
Here, it is known that the aperture for fairing the parallel ray is best located in an image-side focal position of a collimator lens. This is because it is thereby possible to prevent parallel ray to be admitted from being blocked by the aperture even when an optical axis of the collimator lens is deviated from that of the light source. As shown in FIG. 6, in a conventional optical scanning device, a collimator lens 100, a lens holder 101, and an aperture 102 are provided separately, and assembled into a single unit in such a way that the aperture 102 is disposed in an image-side focal position of the collimator lens 100 (see, for example, JP-A-2001-071553).
However, to dispose the aperture 102 in the image-side focal position of the collimator lens 100, as in the optical scanning device disclosed in JP-A-2001-071553, it is necessary to form both the lens holder 101 and the aperture 102 so as to have part longer than the image-side focal distance in the optical axis direction, or form one of the lens holder 101 and the aperture 102 so as to have part longer than the image-side focal distance in the optical axis direction, as shown in FIG. 6. There is a problem that this disadvantageously increases a manufacturing cost of the lens holder 101 or the aperture 102.
Moreover, with this structure, the collimator lens 100, the lens holder 101, and the aperture 102 are provided separately, and assembled into a single unit in such a way that the aperture 102 is disposed in the image-side focal position of the collimator lens 100. As a result, when an out-of-tolerance error or the like occurs, the following problem arises. The aperture 102 is shifted from the image-side focal position of the collimator lens 100, whereby parallel ray to be admitted is blocked by the aperture. This makes it impossible to obtain a light beam having a desired beam spot diameter.